The present invention relates to a method of controlling banknotes.
As is known, banknotes are made from a sheet of such a size as to contain several finished banknotes, and which comprises watermarked regions and/or metal bands, and is fed along a given path along which it undergoes various printing steps, each involving a different printing technique.
The most common printing steps are offset printing, by which images normally representing a coloured background are printed; intaglio or copperplate printing, for printing faces of famous people and numbers representing the value of the banknote; and printing of the serial numbers. Offset printing is performed on both faces of the sheet with no alignment with the edge of the sheet, which therefore cannot be used as a reference by which to acquire the coordinates of offset printed details.
Intaglio printing is performed at high pressure using a plate, and deforms the paper so that the plate is inclined, and the intaglio print itself possibly misaligned, with respect to the offset print. Intaglio printing is performed on one or both faces of the sheet, and may comprise a number of successive prints, each of which may be misaligned with respect to the offset or other intaglio prints. Any deviation of the various successive prints comprises translation along an X axis, translation along a Y axis perpendicular to the X axis, and rotation, all of which are also caused, as stated, by deformation of the sheet in the course of printing steps at different pressures.
Once offset and intaglio printed, the sheet is quality controlled to determine acceptable deviations of the successive prints, and to assign each banknote a pass signal. The serial numbers are then printed on the respective passed banknotes, and the sheet is cut into individual banknotes.
Quality control is frequently performed manually by a checker, and consists in visual inspection to ensure the offset and intaglio prints do not deviate too far from an ideal value, and that there are no colouring errors, i.e. over- or underinked regions, or blurring.
Alternatively, quality control may now also be performed automatically using a television camera, which assigns each pixel a luminance value characteristic of a given surface of the banknote corresponding with the pixel, and compares each luminance value with a respective acceptance range. Automatic control is complicated by numerous factors, foremost of which is determining acceptance ranges enabling accurate control of both colouring and relative deviations, which in turn is complicated by the luminance of each pixel depending on various factors, such as the printed region partly occupying the surface corresponding to the pixel, and the type of ink and paper used.
Determining the acceptance ranges is further complicated by relative deviations of up to a millimeter in the various prints being considered acceptable, and by effective colour control requiring the use of television cameras of such definition that each pixel corresponds to a banknote portion of 0.125xc3x970.125 square millimeters. In relation to the size of the surface assigned to each pixel, an acceptable deviation of one millimeter therefore means the luminance value of each pixel may vary within a very wide range. That is, the surface corresponding to each pixel may be either fully inked or have no ink at all, particularly when the surface portion in question is located at the edge of a figure or number, thus calling for a wide acceptance range. On the other hand, however, a wide acceptance range would fail to provide for effective control by possibly passing banknotes which should be rejected.
It is an object of the present invention to provide a method of controlling banknotes, designed to eliminate the aforementioned drawbacks.
According to the present invention, there is provided a method of controlling banknotes comprising a first print and a second print, said first and second prints being effected at different stages; the method comprising the steps of acquiring a first image of the banknote; comparing said first image with a second image; calculating a relative deviation between the first print and the second print of said banknote; and emitting an error signal in the event the luminance values of the first image fail to fall within respective first acceptance ranges of the luminance values of the second image, or said relative deviation fails to fall within a second acceptance range; said method being characterized by comprising the further steps of calculating deformation of the banknote; and forming said second image as a function of the relative deviation, of the deformation, of a reference third image of the first print, and of a reference fourth image of the second print.
The method according to the present invention is particularly advantageous by enabling colour control of the banknotes with none of the errors caused by relative deviation of the first and second prints and by deformation of the banknote.